Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual die from the finished wafer and packaging the die to provide structural support and environmental isolation.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller die size may be achieved by improvements in the front-end process resulting in die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
FIG. 1 shows a conventional flipchip package-on-package (PoP) structure 10. A flipchip type semiconductor die 12 is mounted to substrate 14 with bumps 16. An underfill material 18, such as epoxy resin, is deposited between semiconductor die 12 and substrate 14. Bumps 19 are formed on the opposite side of substrate 14 for further electrical interconnect. Semiconductor die 20, 22, and 24 are stacked over substrate 26 and covered by encapsulant 28. Semiconductor die 20-24 are electrically connected to substrate 26 with bond wires 30. Substrate 26 is connected to substrate 14 with bumps 32.
The underfill material 18 is deposited from a side of semiconductor die 12 using dispensing tool 34, as shown in FIG. 2a. If underfill material 18 is not evenly and uniformly distributed, or if the underfill material is dispensed with excess volume, then the underfill material can bleed onto contact pads 36 of substrate 26, as shown in FIG. 2b. The underfill bleed-out is particular acute for semiconductor devices with high input/output (I/O) density as the contact pads are typically placed closer to the footprint of semiconductor die 12. The bleeding of excess underfill material 18 over contact pads 36 prevents electrical connection of bumps 32 to the contact pads on substrate 14, which causes defects and reduces manufacturing yield.